Method and apparatus for detecting asperities on magnetic disks using thermal proximity imaging

ABSTRACT

A method and apparatus for mapping the character and location of small surface variations on a planar surface. Energy is supplied to an object in close proximity to the planar surface to thereby raise the temperature of the object. The object is moved with respect to the planer surface substantially constant. A decrease in temperature of the object is detected when it is in proximity to the variation to define the location and character of the variation. The energy supply may be thermal energy or optical energy but preferably is electrical energy which heats a resistive element. Preferably, the object is the magnetoresistive head of a disk drive assembly. The surface may be that of a magnetic recording material. The change in temperature is detected by monitoring the resistance of the magnetoresistive coil of the head. The energy may be supplied in pulses to obtain higher peek temperatures while avoiding mechanical distortion of the object. It is preferred that the object be positioned with respect to the surface so that when that relative motion between the surface and the object occurs, the object does not contact the surface.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to a method and an apparatus formapping the topography of a surface. More particularly it relates to amethod and apparatus for detecting small defects by detecting thepresence and nature of these defects using thermal conduction. Morespecifically, it relates to a method and apparatus for analyzing thesurface of a rotating disk in a direct access storage device, such as amagnetic disk drive.

2. Background Art

In data processing systems, magnetic disk drives are often used asdirect access storage devices. In such devices, read-write heads areused to write data on or read data from an adjacently rotating hard orflexible disk. To prevent damage to either the disk or the read-writehead, it has been recognized for a long time that the surface of thedisk should be very flat and free of any bumps or the like which mightbe contacted by the read-write head. Also, the read-write heads havebeen designed so that they will fly over the surface of the rotatingdisk at a very small, though theoretically constant distance above thedisk, the separation between the read-write head and the disk beingmaintained by a film of air. During its flight, the head undergoescontinuous vibration, pitch and roll as the topography of the diskchanges beneath the head. If the quality of the disk or the read-writehead is poor, occasional rubbing or sharp contact may occur between thedisk and the read-write head, leading to damage to the head or to thedisk, or loss of valuable data, or all of these.

Various attempts have been made to provide increased assurance that suchundesirable contact between a read-write head and a recording disk doesnot occur. Rigid manufacturing and quality assurance specifications forboth the recording disk and the read-write head have been instituted.

Disk inspection for various types of defects, including magnetic,optical and topographic (i.e., delamination, voids inclusions,asperities, etc.) is of critical importance for the increasinglystringent production requirements facing a manufacturer today as smallerdrives store more data. Many methods of inspection to find defects arein use, and many more have been proposed. These include opticaltechniques (fiber interferometry, bulk optic shear interferometry,microISA), magnetic readout (simply screening, HRF, etc.,) andmechanical testing (the so-called PZT glide test, described below). Eachof these techniques may play a role in achieving the goal of thevirtually defect free production of magnetic disks. However, with atightening market and more exacting technical requirements as heads flylower and faster, less expensive and more accurate inspection schemesbecome more significant.

The PZT glide test is disclosed in U.S. Pat. No. 4,532,802 toYeack-Scranton et al. A read-write head is provided with a plurality ofpiezo-electric transducers which produce signals related to its movementas it flies over an adjacently rotating recording disk. By filteringthese signals to determine their spectral components in low, medium andhigh ranges, hard contacts between the head and disk, disk wear orroughness and head movement can be determined. While quite satisfactoryin many respects, this technique depends on contact between theread-write head and the disk, and as a result the heads wear out andcostly replacement is required. In addition, resolution in the radialdirection is limited by the geometry of the head to about 2 mm in theradial direction.

U.S. Pat. No. 4,747,698 to Wickramasinghe et al is directed to aScanning Thermal Profiler. A fine scanning tip is heated to a steadystate temperature at a location remote from the structure to beinvestigated. Thereupon, the scanning tip is moved to a positionproximate to, but spaced from the structure. At the proximate position,the temperature variation from the steady state temperature is detected.The scanning tip is scanned across the surface structure with theaforesaid temperature variation maintained constant. Piezo electricdrivers move the scanning tip both transversely of, and parallel to, thesurface structure. Feedback control assures the proper transversepositioning of the scanning tip and voltages thereby generated replicatethe surface structure to be investigated. While this approach providesexcellent depth resolution, it requires the use of an expensive scanningtip. It also has, in common with the approach illustrated in U.S. Pat.No. 4,532,802 discussed above, the disadvantage that it cannot readilybe utilized on an assembled disk drive.

SUMMARY OF THE INVENTION

It is a principal object of the invention to provide a method and anapparatus for the mapping of asperities on a relatively smooth surfaceby thermal proximity imaging.

It is a further object of the invention to provide a method and anapparatus for doing this mapping at low cost.

It is an additional object of the invention to provide a method forstoring data in the form of mechanical features on a smooth surface andretrieving the data by thermal proximity imaging.

It is yet another object of the invention to do thermal proximityimaging without contacting the asperities on a relatively smoothsurface.

It is still another object of the invention to provide a method andapparatus for performing thermal proximity imaging while keeping thedistance between the thermal proximity sensor and the relatively smoothsurfaces essentially constant.

The present invention provides a method for mapping the character andlocation of small surface variations on a planar surface. The methodcomprises the steps of supplying energy to an object in close proximityto the planar surface to thereby raise the temperature of the object;moving the object with respect to the planar surface while keeping thedistance from the planar surface substantially constant; and detecting adecrease in temperature of the object when it is in proximity to thevariation to define the location and character of the variation. Theenergy supply may be thermal energy or optical energy but preferably iselectrical energy which heats a resistive element. Preferably, theobject is the magnetoresistive head of a disk drive assembly. The changein temperature is detected by monitoring the resistance of themagnetoresistive sensor of the head. The energy may be supplied inpulses to obtain higher peak temperatures while avoiding mechanicaldistortion of the object. It is preferred that the object be positionedwith respect to the surface so that when that relative motion betweenthe surface and the object occurs, the object does not contact thesurface.

The present invention is also directed to an apparatus for detecting thepresence of height variations on a substantially planar surface. Theapparatus comprises an object to be placed in close proximity to theplanar surface; energy means for supplying energy to the object tothereby raise its temperature; means for moving the object with respectto the planar surface so as to maintain the object at a substantiallyconstant distance therefrom; and means for detecting a drop intemperature of the object when it is in proximity to a variation. Theenergy means may supply thermal or optical energy, but preferablysupplies electrical energy to heat the object. The object may be themagnetoresistive head of the disk drive assembly which receiveselectrical energy. The detecting means detects a change in temperatureby monitoring the resistance of the magnetoresistive sensor of the head.Preferably the energy is pulsed in order to obtain higher peaktemperatures and yet avoid mechanical distortion of the object. It ispreferred that the object be positioned during use of the apparatus sothat that it does not contact the surface or the variations in thesurface during relative motion of the surface with respect to theobject.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged, schematic view of a read-write head flying over arotating disk having an asperity thereon.

FIG. 2 is a diagram, partially schematic and partially in block, of anapparatus according to the invention.

FIG. 3A is a plot of the amplitude of the signal from an asperity versusthe bias current to the magnetoresistive head.

FIG. 3B is a plot of the amplitude of a read back signal from a writtenmagnetic track versus the bias current to the magnetoresistive head.

FIG. 4A is a thermal proximity image produced using the apparatus ofFIG. 2.

FIG. 4B is an enlargement of a portion of the image of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 a standard magnetoresistive (MR) head or slider 10mounted on an actuator 11 is used to detect disk asperities. Thegeometry of the slider 10 with relation to the disk 12 and an asperity14 is illustrated. As is well known in the art, disk 12 rotates on aspindle 15, driven by a motor 18. An encoder 17 provides data on therelative rotational orientation of disk 12 with respect to slider 10.

Slider 10 has an air bearing surface facing disk 12. The relative motionof the air bearing surface of slider 10 and the planar surface of disk12 maintains the distance between the slider 10 and disk 12substantially constant. The air bearing surface of slider 10 is designedto provide a constant fly height (for a given rotational disk speed) ofslider 10 above the surface of disk 12. The head temperature is elevatedusing Joule heating of the MR element or sensor object 16. As the headpasses over the disk it reaches some elevated temperature determined bythe heat load and by geometry and thermal considerations. If a particlecauses the gap spacing to temporarily vary, the temperature of thestripe will drop and can be sensed as a momentary spike in the headreadout signal due to the non-zero temperature coefficient ofresistance. The amplitude of the spike is proportional to thetemperature differential maintained in the MR head versus the disksurface and to the thermal properties of the asperity, and dependsroughly as 1/d where d is the head-disk spacing (as opposed to theroughly average fly height).

Even in standard operation, the MR sensor object 16 can be expected torun quite hot, since typical bias currents are on the order of 10 mA,with head resistances of a few tens of ohms. Thus, in a head/sliderweighing no more than a gram, tens of milliwatts in Joule heatingoccurs. The temperature rise can be expected to be significant, and infact proportional to the square of the current. The temperature risewill be determined by this heat flow, balanced by convective and/orconductive losses into the atmosphere and the disk. Further heating canbe supplied with a resistor, and in fact may be desirable in order tobias the magnetic sensitivity to near zero. Typically, MR heads have athermal sensitivity of resistance of 3×10⁻³ /K. By avoiding substantialbias at the frequency used to measure the resistance of the head,magnetic contributions can be nearly eliminated.

Particle size can be estimated from the strength of the thermal signal.The effectiveness of cooling depends on both the width and height of theparticle (or void), and during the scan past the defect a fixed amountof heat energy will flow to the disk surface.

Use of existing MR head technology has several advantages. First, noadditional development need be done, and implementation in a test standcan be achieved with little extra cost. Second, a large knowledge baseexists about MR head properties, so that complete understanding ofthermal response versus magnetic properties can be had at small addedeffort. Third, no modification of the head is required, so thatsignificant costs in replacement heads is avoided (as exists with thePZT glide tester described in U.S. Pat. No. 4,532,802). Fourth,topographic screens and magnetic evaluation can be performed nearlysimultaneously (i.e., sequentially), which is important as a time saver,and for providing new information correlating the two properties. Fifth,this technique can provide higher resolution and less ambiguousinformation about asperities than piezo-based methods. Finally, thetechnique can be used to evaluate disks in assembled head-diskassemblies of disk drives.

Referring to FIG. 2, a system for obtaining and evaluating data fromslider 10 is illustrated. A pre-amplifier circuit 20 provides a biascurrent to the magnetic stripe 16 of slider 10 (FIG. 1). A potentiometer22 connected to a voltage source 24 at one end and ground at the otherend permits adjustments of the bias current. The slider of thepotentiometer is connected to one side MR sensor object 16 of slider 10.The other side of MR sensor object 16 is connected to ground, as is theside of voltage source 24 not connected to potentionmeter 22.

Voltage source 24 may supply direct current, alternating current, orpulses having one polarity or alternating polarities. For the highestsensitivity, and therefore the best resolution of height of an asperity,pulses are preferred. Pulsed operation permits the highest peaktemperatures without overheating the slider 10 so as to cause mechanicaldistortion thereof.

Capacitor 26 and resistor 28 form a high pass filter which passessignals from the slider 10 to the non-inverting input of an operationalamplifier 30. Resistor 32 connected from the output of the operationalamplifier to the inverting input and resistor 34 connected from theinverting input to ground determine the gain of operational amplifier30, in a manner well known in the art. Typically, resistors 32 and 34are selected so that operational amplifier 30 has a gain of 500.

Output signals from amplifier 30 are provided as Y axis inputs to afirst channel of an oscilloscope 36. The same signals are sent to a highpass filter 38 and then to a Y axis input of a second channel ofoscilloscope 36. The signals from a small asperity on the disk aregenerally in the form of a sharp spike having a 3 dB width correspondingto a time of less than 50 microseconds, or typically less than 250microns of disk travel. These spikes are displayed on the first channelof oscilloscope 36. The magnetic data, which typically changes amplitudemuch more rapidly, passes through filter 38 and may be viewed on thesecond channel of the oscilloscope 36.

The signals from amplifier 30 are also supplied to a computer interface40 which includes an analog-to-digital converter, of a type well knownin the art, which converts the analog signals from operational amplifier30 to digital form, for acquisition by a computer 42.

Information concerning the rotational position of the slider 10 withrespect to the disk, provided by shaft encoder 17, which may be a pulsefor every revolution of disk 12 is used as a synchronization input tooscilloscope 36. It is also used as a so-called θ position input. It istherefore supplied to computer interface 40 for eventual use by computer42.

The position of slider 10 in the radial direction with respect to disk12 is determined by a head position sensor 43 associated with theactuator 11. This radial position information is also supplied tocomputer interface 40.

The information supplied to computer interface 40 provides threedimensional data where the θ and radial position data define theposition of an asperity on the disk 12, while the information derivedfrom the output of amplifier 30 provides an indication concerning theseverity of the asperity, with respect to height. The information isstored in a data base in computer 42, processed by suitable processingtechniques and finally displayed on a display screen 44. Alternatively,a hard copy print out is provided by a printer 46.

FIG. 3A, is a plot of the thermal signal output from a defect as afunction of current. The amplitude is parabolic in current and suggeststhat the amplitude of the signal is proportional to the power dissipatedin the head and therefore to the temperature difference between theslider 10 and the disk 12.

FIG. 3B, illustrates the MR read signal amplitude from a written trackon a disk. The "S" shape is characteristic of the MR head showingincreased sensitivity with current into a linear region, followed bysaturation at high current. Thus "magnetic contrast" or the influence ofmagnetic domains may be separated from the topographic information byoperating at two different currents. Alternatively by using an ACcurrent and observing the signals produced at the second harmonic,magnetic contrast can be removed. Since magnetic signal strength isnominally linear with respect to MR bias current, magnetic and thermalinformation can be separated in the following way: Instead of using a DCbias, a current at some frequency fo is provided. Then any signalmeasured from the MR element at a frequency 2 fo is due to the thermalvariations and not to magnetic information.

Further, the shape of the signals produced is quite different. Smallasperities produce a characteristic spike shape, as noted above, whilemagnetic variations on the surface of the disk produce extended widthsignals. Thus, filtering, as described above, is also a method ofdistinguishing magnetic signals from thermal signals.

FIG. 4A, is a thermal proximity image produced by a method and apparatusaccording to the invention. FIG. 4B, is an enlarged view of the lowerright hand corner of FIG. A. The disk used to make the images was madeof aluminum with no magnetic layer deposited thereon. However, the diskwas carbon coated and a lubrication coating was applied. The array oflines that can be seen on the disk were provided by a lithographytechnique. These lines have a width of between 10 and 100 microns, andheights between 200 and 800 Å.

The light regions are unintended asperities having widths ofapproximately 100 microns and heights above 800 Å.

It is contemplated that with an improved, specially designed sensor andan improved system for data acquisition and analysis, resolution in thecircumferential direction can be improved to be in the range of 2 to 5um. With respect to height, the Johnson noise limits the resolution tobelow one Å. for a bandwidth of 1 MHz.

While the description of the invention set forth above has centeredprimarily on the mapping and characterization of asperities, it is notedthat the invention may also be applied to a method and apparatus thestorage of information. In particular, information can be encoded intothe surface of a disk in the form of small raised or lowered portions.Using the thermal imaging method and apparatus for described herein, theinformation can be read back and retrieve for use in, for example, adata processing system. Further, in view of the ability to discriminatebetween magnetic signals and signals resulting from the topography ofthe disk, it is possible to encode some information on the disk usingmagnetic techniques and other information using topographicaltechniques. Different kinds of information can be encoded in this manneras, for example, it may be preferable to encode information to bepermanently stored by topographical techniques while information whichis to be changed can be encoded by magnetic techniques.

While the invention has been particularly shown and described withrespect to a preferred embodiment thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for mapping the character and locationof surface variations of relatively small height on a substantiallyplanar surface comprising the steps of:flying a slider over the planarsurface at a substantially constant fly height, said slider carryingsensor object in close proximity to said planar surface; supplyingenergy to said object to thereby raise its temperature; and detecting adecrease in temperature of said object when it is in proximity to asurface variation to define the location and character of the variation.2. The method of claim 1 wherein electrical energy is supplied to saidobject in said supplying energy step.
 3. The method of claim 1 whereinsaid slider is a magnetoresistive head of a disk drive assembly and saidobject is a magnetoresistive sensor in said head.
 4. The method of claim3 wherein the energy is electrical energy which is supplied to themagnetoresistive sensor of the head.
 5. The method of claim 4 whereinthe decrease in temperature is detected by monitoring the resistance ofthe magnetoresistive sensor.
 6. The method of claim 1 wherein saidobject does not contact said surface or said surface variations.
 7. Themethod of claim 1 wherein said slider has an air bearing surface, andrelative motion of the air bearing surface and the planar surfacemaintains said slider at a substantially constant fly height above saidplanar surface.
 8. The method of claim 1 further comprising the steps ofacquiring data concerning the location of said variations on saidsurface, and creating a display of temperature variation of the objectas a function of location.
 9. The method of claim 1 wherein said surfaceis a surface of a magnetic storage medium, and said variations areasperities on said surface.
 10. A method for mapping the character andlocation of surface variations of relatively small height on asubstantially planar surface comprising the steps of:supplying energy toan object in close proximity to the planar surface to thereby raise itstemperature; moving the object with respect to the planar surface whilekeeping the object a substantially constant distance from the planarsurface; and detecting a decrease in temperature of the object when itis in proximity to a surface variation to define the location andcharacter of the variation, wherein pulses of energy are supplied to theobject to obtain peak temperatures and to avoid mechanical distortion ofthe object.
 11. A method for detecting the presence of asperities on aplanar surface comprising the steps of:supplying energy to an object inclose proximity to the planar surface to thereby raise its temperature,said energy being supplied in the form of pulses of sufficient magnitudeto increase peak temperature and avoid mechanical distortion of saidobject; moving the object with respect to the planar surface whilekeeping the object at a substantially constant distance from the planarsurface; and detecting a change in the temperature of the object when itis in proximity to an asperity.
 12. An apparatus for detecting thepresence of surface variations on a planar surface comprising:a slider;means for flying said slider over the planar surface at a substantiallyconstant fly height; a sensor object carried by said slider in closeproximity to said planar surface; energy means for supplying energy tosaid object to thereby raise its temperature; and means for detecting adrop in temperature of the object when it is in proximity to a surfacevariation.
 13. The apparatus of claim 12 wherein electrical energy issupplied to said object by said energy means.
 14. The apparatus of claim12 wherein said slider is a magnetoresistive head of a disk driveassembly and said object is a magnetoresistive sensor in said head. 15.The apparatus of claim 14 wherein said energy means supplies electricalenergy to said magnetoresistive sensor of the head.
 16. The apparatus ofclaim 15 wherein the detecting means detects a change in temperature bymonitoring resistance of the magnetoresistive sensor.
 17. The apparatusof claim 14 wherein said planar surface is a surface of a magneticstorage disk and said means for flying the slider with respect to theplanar surface includes a spindle on which said disk is mounted and amotor for rotating said spindle.
 18. The apparatus of claim 12 furthercomprising:means for acquiring data concerning the location of saidvariations on said surface, and means for creating a display oftemperature variation of said object as a function of location.
 19. Anapparatus for detecting the presence of surface variations on a planarsurface comprising:an object to be placed in close proximity to saidplanar surface; energy means for supplying energy to said object tothereby raise its temperature; means for moving the object with respectto the planar surface so as to maintain the object at a substantiallyconstant distance therefrom; and means for detecting a drop intemperature of the object when it is in proximity to a surfacevariation, wherein the energy means supplies pulses of energy ofsufficient amplitude to obtain high peak temperatures and to avoidmechanical distortion of said object.
 20. An apparatus for detecting thepresence of surface variations on a planar surface comprising:an objectto be placed in close proximity to said planar surface; energy means forsupplying energy to said object to thereby raise its temperature; meansfor moving the object with respect to the planar surface so as tomaintain the object at a substantially constant distance therefrom; andmeans for detecting a drop in temperature of the object when it is inproximity to a surface variation, wherein said energy means includes anadjusting means for adjusting an amount of energy supplied, whereby saidapparatus may be used to distinguish between magnetic and topographicsurface variations.